Variable displacement rotating pumps are well known in the prior art. Such pumps are often used in hydraulic systems to provide fluid power to components such as hydraulic cylinders and rotary actuators. An exploded view of a typical variable displacement rotating piston pump is shown in FIG. 1.
The pump generally indicated 10 includes a case 12 which has a first section 14 and a second section 16. A plurality of movable pistons 18 are mounted inside the case in a carrier 20. A spring inside carrier 20 biases multiple pins 15 against a ball guide 17. The ball guide pushes against a slipper plate 19. The slipper plate 19 biases the pistons away from the carrier. The carrier and pistons are rotatable inside the case when driven by a drive shaft 22.
A swash plate 24 is mounted inside the pump case. A wear plate 26 is positioned on the swash plate when the pump is assembled. As later explained, when the pump is operated, the pistons 18 ride on the wear plate 26. The swash plate is mounted to the case by a pair of mounting pins 28 which extend into mounting holes 30 in the first section of the case. Bearings 34 support the pins in the mounting holes, and retaining rings 36 keep the bearings and pins from moving laterally inside the case. The mounting of the swash plate 24 enables it to swivel about an axis perpendicular to the axis of rotation of shaft 22 and pistons 18.
A balancing spring 38 is mounted in the pump case. A spring guide 40, positioned on spring 38, contacts swash plate 24 to bias it in a first direction. A servo piston 42 is mounted on the second section 16 of the case. Servo piston 42 contacts swash plate 24 on a side opposite spring guide 40.
A fluid directing plate 44 is mounted adjacent to piston carrier 20 and directs fluid into inlet and outlet passage 46 and 48 respectively, in the second section 16 of the pump case.
The operation of the variable displacement rotating piston pump is now explained with reference to FIG. 2. Fluid is delivered to the pump through an inlet 50 in case 12. The inlet 50 is connected to inlet passage 46. Fluid in the inlet passage flows into the pistons 18 when they are located in the lower portion of the pump as shown in FIG. 2. When servo piston 42 is in the retracted position as shown in FIG. 2, swash plate 24 is tilted at an angle by the force of spring 38.
The pistons 18 include ball shaped slippers 52 which swivel. The ball shaped slippers also include a small fluid passage 54. A small amount of fluid flows to the bottom of the ball shaped slippers through passages 54 which enables the piston assemblies to slide on wear plate 26 with minimum friction.
When shaft 22 rotates, it rotates carrier 20 and the pistons 18. As shown in FIG. 2, because swash plate 24 is tilted, the fluid is pushed out as the pistons approach the upper portion of the pump case and fluid flows out of outlet passage 48. As a result, fluid is delivered from the pump at an outlet 56. Fluid is pulled into the pistons when they are pulled away from the fluid directing plate 44 as they pass through the opposite area of their rotational path. As can be seen in FIG. 2, the greater the angle of swash plate 24, the larger the volume of fluid pumped at a given rotational speed of the shaft.
Fluid power systems typically operate at variable pressures. This is because the devices that perform the work, a hydrualic cylinder for example, often encounter variable resistance to movement. A log splitter which operates using a hydraulic cylinder is an example of this phenomenon. The wedge which contacts and splits the log is attached to the cylinder. Until the wedge contacts the log, the cylinder moves the wedge with little resistance. As a result, pressure of the working fluid in the cylinder is low. When the wedge contacts the log, the resistance to further movement (and the pressure inside the cylinder) builds rapidly. Once the log fractures, the resistance force drops and the corresponding pressure in the cylinder drops as the wedge continues to move against lesser resistance.
If a piston pump with a fixed displacement were used to power the hydraulic cylinder of a log splitter or other device that encounters variable force, the amount of power required to drive the pump during the high pressure periods would be very high. This, a very large motor would be required. Further, if the power required to drive the log spitter or other device became higher than the motor could deliver, the motor would stall and the pump would stop.
Variable displacement rotating piston pumps can be used to minimize these problems. This is accomplished by varying the angle of the swash plate. When the pressure in the system rises, the flow through the pump is reduced. This maintains the amount of power the motor driving the pump must supply within a manageable range.
A prior art system which reduces the flow through the pump at high pressure is shown in FIG. 2. This system involves use of a first compensator valve assembly 58. Valve assembly 58 has a body which houses a first internal chamber 62 and a second internal chamber 64. A compensator spool 66 is movably mounted in the first internal chamber 62. A pre-load spring 68 is mounted in the second internal chamber 64. The pre-load spring 68 biases compensator spool 66 to the left as shown in FIG. 2. The biasing force is set by turning an adjusting nut 70 which is attached to an adjusting rod 72 threaded in body 60.
First chamber 62 is in fluid communication with outlet passage 48 through a fluid passage 74. First chamber 62 is also in fluid communication with the interior of servo piston 42 through a fluid passage 76.
The pressure at the outlet 56 of the pump rises when the fluid power system supplied by the pump increases its working pressure. When this occurs, the pressure correspondingly increases in chamber 62 and attempts to push the compensator spool toward the right. If the outlet pressure rises high enough to overcome the force of pre-load spring 68, the compensator spool will move to the right of the position shown. When the spool moves, fluid pressure from chamber 62 is delivered to fluid passage 76 and into the interior of the servo piston 42. The servo piston moves to the right overcoming the force of spring 38. When the servo piston extends, the angle of the swash plate decreases. This reduces the volume of fluid flowing through the pump. As a result, the motor driving the pump does not have to provide as much power. This is because the pump is delivering a lesser volume of fluid at the elevated pressure.
When the pressure at outlet 56 drops, pre-load spring 68 moves the compensator spool back to the left. Fluid in the servo piston is pushed back through flow passage 76 into first chamber 62. The fluid then passes through a fluid passage 78 into a low pressure area inside the pump case. When the fluid pressure in the servo piston is relieved, the piston retracts and the volume of flow through the pump increases.
A problem with this system is that it cannot take full advantage of the power available from a particular motor. This is because the compensator valve must be preset to lower the flow whenever a fixed pressure is exceeded. The power delivered by a piston pump is a function of both volume and pressure. As this compensator valve assembly works on pressure only, it cannot take full advantage of the power available.
Another type of prior ar control valve for controlling the operation of a variable displacement rotating piston pump is shown in FIG. 3. This system includes a second compensating valve 80 which has a body 82. Body 82 includes first, second and third internal chambers 84, 86 and 88 respectively, which are connected. First chamber 84 is in communication with outlet passage 48 of the pump through a fluid passage 90. Second chamber 86 is connected to servo piston 42 of the pump through a fluid passage 92. Third chamber 88 is connected to a fluid passage 94 which extends through body 82. Fluid passage 94 extends through a fourth chamber 96 to a control port 98.
A spool 100 is moveably mounted in the valve body and extends through the first, second and third chambers. Spool 100 includes an orifice passage 102 which enables fluid to pass from the first chamber 84 to the third chamber 88 through the interior of spool 100. A differential spring 104 biases the spool to the left as shown in FIG. 3.
In operation of the second compensator valve 80, the flow through the pump (and thus the power required to drive the pump) may be controlled by varying the pressure at control port 98. The pressure delivered at the outlet 56 of the pump is communicated to first chamber 84 through fluid passage 90. The fluid pressure in the first chamber 84 is bled off to third chamber 88 through orifice passage 102 in spool 100. In the position of the spool shown in FIG. 3, no fluid is delivered to the servo piston 42 which is shown in its fully retracted position.
When the pressure at pump outlet 56 exerts a pressure on spool 100 which exceeds the biasing force of the differential spring 104 plus the controlled fluid pressure at control port 98, the spool moves to the right of the position shown in FIG. 3. When this occurs, fluid delivered to the first chamber 84 is enabled to pass into the servo piston 42 through the second chamber 86 and flow passage 92. As the servo piston extends, the angle of the swash plate 84 is reduced and the volume of fluid flow through the pump drops.
When the pressure at the outlet 56 falls, (or the control pressure at control port 98 increases) so that the forces pushing spool 100 to the left are greater than the pressure at the outlet port pushing it to the right, spool 100 moves back to the position shown in FIG. 3. When this occurs, fluid in servo piston 42 flows back into the second chamber 86 through flow passage 92. Then the fluid in the second chamber 86 flows into the case through a flow passage 106. As fluid leaves the servo piston it retracts, and the flow through the pump increases.
Although the system described above provides for variable control of the servo piston of the pump, there is a need to provide a pressure relief control to be sure the maximum pressure capability of the pump is not exceeded. This control is provided by a pressure relief valve portion generally indicated 108. The pressure relief valve portion includes an adjustable rod 110 which extends through forth chamber 96. The valve is threaded and the valve body and its position may be changed by rotating an adjusting nut 112. Rod 110 has an internal fluid chamber 114 which is open to fourth chamber 96 as shown.
A dart 116 is adjacent the opening to internal fluid chamber 114. A spring 118 biases the dart to close the opening When the force of spring 118 is exceeded by the force of the fluid in fourth chamber 96, the dart is pushed to the left and relieves pressure through a fluid passage 120 to second chamber 86. Fluid passage 120 is positioned so fluid therefrom is always passed to the case regardless of the position of spool 100. Relief valve portion 108 provides a fixed maximum pressure that can be held at control port 98, and thus the maximum pressure that can be produced at the outlet port of the pump before the servo piston moves to reduce flow.
The prior art construction of the second compensating valve is useful in that it provides for variable control of the volume of flow through the pump. However, it does not solve a significant problem associated with variable displacement rotating piston pumps. That problem is to control the volume flow through the pump in relation to the outlet pressure so that the power producing capabilities of a motor which is used to drive the pump are not exceeded. At the same time it is also necessary to fully utilize the power available from the motor. Thus there exists a need for a device which achieves these results.